Key Engineering Materials Vol. 813

Abstract: NiCrBSi coatings were produce by Flame Spray on a carbon steel substrate. The “as spray” coatings were refused by means of Pulsed Gas Tungsten Arc Welding (GTAW-P) process, following different patterns and welding procedure, with objective of optimize the coating characteristics and productivity. The patterns evaluated were oscillated triangular (OT) and oscillated sinusoidal (OS). Travel speed and workpiece-electrode distance were also analyzed. On each obtained sample the surface appearance, macro and microstructure on transverse cuts were evaluated, determining penetration, dilution and level of defects, among others features. Microhardness profiles and adherence were also evaluated. The OS pattern show a more uniform profile of the refused thickness, with less dilution with the base material. Travel speed and workpiece-electrode distance have both significant effect on the volume of refused material, affecting dilution and consequently the resultant coating hardness.

Abstract: In this paper, we report on synthesis of graphene film on Cu foil by cold wall CVD and successfully transferred to a photovoltaic cell. The obtained sample was covered with an ultra-thin layer of Ni, of about 4 nm, using a sputtering technique. The optical and electrical properties of graphene/Ni-based films showed superior performance (transmittance =65%, sheet resistance=250 Ω/sq; EQE=40%) compared to films made of ITO/nickel, described in literature, of greater thickness.

Abstract: We present the results obtained using Cr₂O₃ as selective absorbing layer on aluminium film substrate. The thin films were deposited by electron beam evaporation using a multiple crucible e-gun able to deposit 4 materials in sequence without breaking the vacuum. Optical characterization of the multilayer films is based on ellipsometry and hemispherical reflectivity. The complex refractive index has been determinate and it has been used to design a selective solar absorber with high absorptance.

Abstract: In the present work, the surface of Ti-6Al-7Nb samples was patterned with Laser Induced Periodic Surface Structures in order to improve biocompatibility, increase tissue ingrowth and decrease bacterial adhesion and inflammatory response for applications in dental and orthopedic implants. Polished and sandblasted disks 10 mm in diameter were treated generating LIPSS under two different sets of parameters. The surface morphology and chemistry were investigated both by secondary electrons imaging, EDS analysis and Atomic Force Microscopy. Primary rat osteoblast culture (passage 2) was used to assess cell toxicity and biocompatibility. Alamar Blue assay was used to access cell viability and proliferation on day 1, 3 and 7. The difference between cell adhesion on polished and sandblasted surface as well as between polished and LIPSS-modified surface are described and discussed.

Abstract: Ti alloys have been intensely used for human implants due to its excellent characteristics, like bio-inertness, low density, and corrosion resistance. However, some alloying elements were found to be toxic for the human body, which restricts the use of some alloys. Furthermore, there are two additional and essential aspects to be considered. The first relates to the young modulus that, despite being lower than other alloys commonly used for this purpose, it is still far over from the human bone modulus. Such high modulus can result in the stress shield phenomena and the consequent implant losing. The second aspect relates to the fact that bio-inertness does not guarantee a complete tissue integration to the implant and, consequently, the expected implant performance. In this context, new low modulus b-Ti alloys containing nontoxic elements have been developed in recent years, and several surface modification processes have been proposed to promote better implant/tissue integration.In the present work, the new b-type Ti-Mo-Zr-Fe alloy has been submitted to a plasma enhanced chemical vapor deposition (PECVD) process in order to form a superficial titanium nitride layer, aiming to produce a satisfactory substrate for the tissue cells growing. In a first step, microstructural characterization and corrosion performance of the modified alloy surface has been evaluated by Electrochemical Impedance Spectrometry and Potentiodynamic testing, and the results compared to the unmodified alloy. It was found that during the plasma nitriding process, that runs at 550°C for 1h, the metastable b microstructure is partially converted into a’ and possibly a” phases, which can impact the young modulus. The 500nm thick TiN layer formed over the alloy surface improved the corrosion behavior of the alloy. These results encourage the continuity of the research, with the future in vitro bio-activity testing of the nitrided surface.

Abstract: In the present work the performances of hard anodized component in Al-Si alloy, used as cooker grids, are described in details. The components have been anodized in H₂SO₄ at low temperature (less than 0 °C) with a current of about 2.4/dm² A for 70 min.The effect of the alloy microstructure on the quality of the anodized layer is evidentiated, particularly the Si rich intermetallics inside the metal have a detrimental effect on the performance of the oxidized layer.The components have been analyzed by means of nano-indentation to evaluate the mechanical behavior of the layer. The chemical performances have been studied using Electrochemical Impedance Spectroscopy in different solutions (0.05 M Na₂SO₄ and 0.01 M NaOH). The results obtained have been correlated with the microstructure of the alloy, furthermore all test have been done, for comparison, on Commercially Pure Aluminum anodized in the same conditions of the Al-Si components.The results indicate that the mechanical properties of the anodized layer of Al-Si components are lower to respect that of Commercially Pure Aluminum.On the contrary the chemical resistance of Al-Si anodized items result poor compared to Commercially Pure Aluminum, in particular the oxidized layer is subject to degradations due to the presence of Si rich inclusions.

Abstract: This work aims to study the wear resistance of composite materials that mainly differ in the fibre typologies used as reinforcement. In detail hemp, glass and carbon fibres in form of woven fabric were used. For the production of the composite materials, an epoxy resin was used as matrix, and the vacuum infusion process was adopted. In order to compare the tribological behaviour of the manufactured composites, a detailed experimental campaign, including tribological tests and microgeometrical measurements, was carried out. In particular, the tribological behaviour was studied through the pin-on-disk tests conducted at 210 mm/s as peripheral speed under 50 N as applied load testing both the composite and the single un-impregnated fabrics. These tests were followed by microgeometrical measurements in order to critically observe the wear tracks, evaluate their depth, width and volume and then to calculate the final less of volume. The tests results showed a good and interesting behaviour of composite materials reinforced with hemp fibres.

Abstract: In the last decades, structural adhesives have found an increasing use owing to the fact that they, compared to traditional techniques like bolting and riveting, allow more uniform stress in the joint without drilling the parts of the assembly. Moreover, the quality of a bonded joint is strongly linked to the surface treatment used for the adherends. Recently, many studies showed the opportunity to use a CO₂ laser for enhancing the mechanical properties of bonded joints in FRP. Nowadays, to optimize their process parameters and texturing represent a necessary step for the growth in this technology. In this work, a numerical study of the effect of laser texturing on end notched flexure (ENF) joints realized in CFRP has been developed. In particular, it is possible to design of the laser texturing for optimizing the mechanical response of bonded joints.

Abstract: The aim of the present work was the assessment of the impact of deep-rolling and shot-peening performed in the underhead and in the unthreaded shank of two high strength screws (36 NiCrMo and 42 CrMoV) for fatigue life enhancement. The experimental campaign consisted of six combinations, including the non-treated state. The aforementioned treatments were evaluated alone or with shot-peening performed after deep-rolling in the underhead fillet of the screws. Deep rolling was carried out at the optimal rolling force, whereas two shot diameters were considered for shot-peening (Z100 and UFS70). The results have been evaluated in terms of fatigue limits and factor effects have been assessed by marginal mean plots.

Abstract: Porosity is a crucial property in cold sprayed coatings. If it can be beneficial for certain applications, e.g. lubricated contacts in tribology, it is generally detrimental for mechanical and anti-corrosion properties and for gas tightness of the coating. It is thus important to understand what are the process variables affecting coating porosity and quantify their effects. However, at present day, the experimental observation of cold spray coating build up and porosity creation mechanisms is far from achievable. In this study, a Coupled Eulerian Lagrangian (CEL) numerical 3D finite element analysis of single and multi-particle impacts was led to understand the mechanisms involved in the creation of porosity. All simulations were carried out on Abaqus/Explicit v6.14. Single particle impact simulations addressed the influence of mesh size, particle temperature, size and speed on particle temperature and equivalent plastic deformation after impact. Jetting phenomena appeared and their relationship to the mesh size was established. Nevertheless, the porosity creation mechanisms seemed to arise within the more complex context of multiple particle interactions at impact. For this reason, multiparticle impact simulations were carried out, based on experimental powder granulometry distributions and particle in-flight velocity measurements. Porosity in these simulations was quantified by computing the successive layered convex hulls of the particle cluster after impact. Individual particles were also tracked during the simulations, in order to understand the porosity creation mechanisms. Finally, those porosity rates were compared to those experimentally measured in 2D by SEM image analysis.